The present invention relates to a process for treating foamed sheets on a polyvinyl chloride (PVC) basis to improve the embossing properties of the foamed sheets.
In the production of foamed sheets on a PVC basis the top side of the foamed sheet is treated in a separate operation with a lacquer solution in order to give an after-treatment to the surface of the foamed sheet. This after-treatment, which is also known as surface finish, is intended to give the sheet surface particular properties, such as dryness or dirt repellancy, and also to create a barrier effect against the emigration or exudation of plasticizers and to give the sheet a generally pleasant appearance.
Hitherto, lacquer solutions of a resin in a low-boiling lacquer solvent have been used, such as, for example, polymethyl methacrylates in combination with PVC, which may if necessary contain matting agents such as silicon dioxide or finely divided polyethylene waxes to matt the lacquer and also tin stabilizers. Generally the lacquer solutions consisted of 10 to 20% by weight of a resin, the remainder being low-boiling lacquer solvent such as, for example, methyl-ethyl ketone.
The foamed sheets with a surface coating of such a lacquer solution were then dried to remove the solvent at temperatures between 80.degree. and 100.degree. C., a lacquer film being formed by the evaporation of the solvent. After the film had formed, the foamed sheet thus treated was embossed at temperatures of over 200.degree. C.
A considerable disadvantage of these conventional processes is that due to the drying speeds of over 20 m/minute required to remove the low-boiling lacquer solvent the latter cannot be completely removed, and a quantity of the solvent (for example, 3 g/m.sup.2) is left in the foamed sheet; this cause bubbles during the subsequent embossing operation, the result being an unsightly surface.
It is an object of the present invention to improve a process for enhancing the embossing properties of such foamed sheets on a PVC basis and to propose the use of a lacquer solution by which a better surface is obtained.